i18n-tasks is a gem to find and manages missing and unused translations
in your
application.

The default approach to locale data management with gems such as i18n is
flawed. If you use a key that does not exist, this will only blow up at
runtime. Keys left over from removed code accumulate in the resource
files and introduce unnecessary overhead on the translators. Translation
files can quickly turn to disarray.

i18n-tasks improves this by using static analysis. It scans calls such
as I18n.t(‘some.key’) and provides reports on key usage, missing, and
unused keys. It can also can pre-fill missing keys, including from
Google Translate, and it can remove unused keys as well.

i18n-tasks can be used with any project using i18n (default in Rails),
or similar, even if it isn’t ruby.